Buckypaper made with carbon nanotubes derived from CO2

Unusual buckypapers, sheets of graphene nanocarbons (GNCs) such as carbon nanotubes, were formed with GNCs directly derived from CO2via molten carbonate electrolysis. Examples are presented for buckypapers made from CO2 using either crushed or chemically washed GNCs, epoxy-infused CNTs, or GNCs pressed directly using a hot electrolyte to remove excess electrolyte.

In 2015, it was shown that the growth of transition metal nuclei during this electrolysis process leads directly to the conversion of CO 2 into pure graphene nanocarbons, including carbon nanofibers and carbon nanotubes (CNTs) [6].This transformation of the greenhouse gas CO 2 into valuable GNC products offers a chance to convert CO 2 into a form of carbon stabilized by graphene, thus aiding in mitigating climate change.Graphite is an analogous macroscopic form of layered graphene, and as a mineral graphite has an established geologic (hundreds of millions of years) lifetime.
The CO 2 to nanocarbon process, including electrolyte separation and return to the electrolysis chamber, and extraction of the pure GNC product is illustrated Figure S1.As illustrated in Figure 1, during electrolysis CO 2 either sourced directly from the air or industrial emissions are transformed to GNCs by electrolysis in molten carbonates.The CO 2 is split into C and O 2 with a GNC-electrolyte matrix growing at the electrolysis cathode.This 1 Supplementary Information (SI) for RSC Advances.This journal is © The Royal Society of Chemistry 2024 nanocarbon/carbonate electrolyte mix has been termed a carbanogel and is refined through the separation of the electrolyte. .
The high electrical conductivity character of the graphene nano-allotropes supports continuous growth during the CO 2 molten electrolysis at low electrolysis voltage.This cathode product grows as an interconnected matrix with electrolyte in the matrix pores.This matrix containing carbonate electrolyte has been termed a carbanogel.Some of the electrolyte in this matrix is rather loosely bound.For example, a post-electrolysis cathode lifted out of the molten electrolyte can release over 30% of the bound electrolyte by gravitational drip.
Figure S2 illustrates larger vertical presses that have been scaled up, which include the transfer of applied pressure to the pressing chamber using a hydraulic ram, as described previously [9].In Figure S2A, there's a cross-sectional depiction of an intermediate scaled-up carbanogel electrolyte extraction unit, detailing the plunger, filter screen platform, and electrolyte exit chamber [9]. Figure S2B   Control of the electrode and electrolyte composition, and CO 2 electrolysis splitting temperature and current density tunes the decarbonization process to form a range of high purity graphene nanocarbon products, including carbon nanotubes.Typical SEM, TEM and HAADF (High Angle Annular Dark-Field TEM) elemental analysis imaging of the CNTs are presented in Figure S3, and have been extensively detailed [7].
Control of the CO 2 electrolysis conditions is used to tune the specific GNC generated by control of the temperature, current density, and the composition of the electrolyte [8].For example, a lower temperature (725°C) is typically used in the electrolytic growth of carbon nano-onions, while higher temperature (750 to 770°C) is used in the electrolytic growth of carbon nanotubes.Lithium carbonate, a typical electrolyte, has a melting point of 723°C.Binary lithium carbonate mixtures have a lower melting point.A high sodium carbonate content in a mixed sodium/lithium carbonate electrolyte and a lower electrolysis temperature (670°C) drive the formation of a graphene scaffold nanocarbon product formation.Applied electrolysis current densities generally range from 0.03 to 0.6 A cm -2 .High current density (0.6 A cm -2 or over) is one of the principal conditions driving the formation of fascinating helical, rather than straight, carbon nanotubes.
Electrode (and electrolyte additive) composition variation has been used to grow other GNC allotropes from CO 2 .These include carbon nanobamboo, carbon nanopearl, graphene from nanocarbon platelets, carbon nanofiber, carbon nanobelt, carbon nanotree, and other specific carbon allotrope morphologies.SEM of a range of these GNC products is presented in Fig. S4, and XRD and Raman spectra of the products are presented in Figs.S5 and S6 as previously detailed [8].The solid graphene nanocarbon product from CO 2 grows as a matrix directly on the cathode.Under constant current electrolysis conditions, the product formation is continuous, and the growth occurs in the direction towards the anode.
shows a larger carbanogel electrolyte extraction unit in operation, capable of pressing up to 0.25 tonnes of carbanogel.Presses in the unit with 50 kg carbanogel have already achieved over 99% electrolyte extraction efficiency.

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Figure S3.SEM TEM and HAADF of the synthesis product of high purity, high yield carbon nanotubes by electrolytic splitting of CO 2 in 770°C Li 2 CO 3 .The SEM has a scale bar of 5 µm.Panels B are TEM with scale bars decreasing from 100, 20, 5 and 1 nm.Bottom rows panels C are HAADF elemental analyses with scale bars decreasing from 100 to 50 nm, and in the bottom right a HAADF elemental carbon profile analysis of the carbon nanotube cross section.Modified from open access paper X.Liu, G. Licht, S. Licht, Controlled Transition Metal Nucleated Growth of Carbon Nanotubes by Molten Electrolysis of CO 2 Catalysts 12 (2022) 137.https://doi.org/10.3390/catal12020137.

Figure S5 .
Figure S5.XRD of the synthesis product consisting of various labeled unusual nanocarbon morphologies synthesized by the electrolytic splitting of CO 2 in 770°C Li 2 CO 3 with a variety of systematically varied electrochemical conditions.Modified from open access paper X.Liu, G. Licht, X. Wang, S. Licht, Controlled Growth of Unusual Nanocarbon Allotropes by Molten Electrolysis of CO 2 .Catalysts 12 (2022) 137.https://doi.org/10.3390/catal12020125.